The invention relates to a novel method for spot-knocking the electron gun mount assembly of a CRT (cathode-ray tube), and more particularly to a method of spot-knocking an electron gun mount assembly utilizing an axial magnetic field.
In the manufacture of a CRT, it is the practice to electrically process the electron gun mount assembly therein after the CRT has been completely assembled, exhausted of gases and sealed. One step in this electrical processing is spot-knocking, which involves inducing arcing in the gaps between adjacent electrodes, usually between a focus electrode and an electrode adjacent thereto. Arcing removes projections, burrs and/or particles which would later be sites for the field emission of electrons during the normal operation of the CRT. One problem encountered in this process is the initiation of arcs in the CRT in locations which by-pass the primary gaps between the adjacent electrodes. Typical of such unwanted arcing are arcs transverse to the electron gun longitudinal axis, such arcs occur between the elements of the electron gun and the neck glass of the CRT envelope.
U.S. Pat. No. 4,214,798 issued to L. F. Hopen on Jul. 29, 1980 discloses a conventional spot-knocking method that may be applied to bipotential or a tripotential electron gun structures. A bipotential gun structure typically has a heater and cathode K, a control grid G1, a screen grid G2, a single focus electrode G3 and a high voltage electrode, which is often designated as the anode or G4. Although separate elements may be provided for each of the three electron guns of a color picture tube, recent practice has tended to use common elements for the G1, the G2, the G3 and the anode of the three electron guns. A tripotential gun differs from a bipotential gun in that it employs three focus electrodes for the focusing action instead of only one. A tripotential gun typically has a heater, a cathode K, a control grid G1, a screen grid G2, three focus electrodes G3, G4 and G5, and an anode, which is often designated G6. In the method described in the Hopen patent, the heater, the cathode, the control grid and the screen grid are interconnected and, in the bipotential gun structure, spot-knocking voltages are applied between the anode and the interconnected gun elements with the focus electrode electrically floating. The tripotential electron gun is similar to the bipotential electron gun for the purpose of spot-knocking except that the G3 and G5 focus electrodes are interconnected within the CRT and two separate stem leads are connected to the G3 and G4 focus electrodes which are electrically floating during spot-knocking.
Many methods of spot-knocking electron gun assemblies have been used previously in attempts to improve the electrical characteristics of television pictures tubes. Most of these methods involve forcing arcs to occur between two adjacent electrodes to remove projections, burrs, and/or particles so that the field emission of electrons between the two elements is significantly reduced at the normal operating potentials. In all cases involving spot-knocking between the anode and the focus electrode G3, positive fluctuating DC high-voltage pulses are applied between these two electrodes with all other electrodes being held at ground potential or allowed to float, as described in the above-referenced Hopen patent. An alternative is to ground the anode and apply negative fluctuating DC high-voltage pulses to the remainder of the gun structure. The size, shape and repetition rate of the high-voltage pulses vary widely depending upon the nature of the spot-knocking equipment used. The voltage pulses used most frequently for spot-knocking are sinusoidal and are derived from the normal variation of the line voltage. They may be half wave with the lowest portion either at some minimum positive DC level or at ground potential, or they may be full wave, in which case the lowest value is usually clamped at ground potential. Very fast rise time pulses of short duration, sometimes derived from the discharge of a capacitor through a ball gap, have also been used in which current pulses often exceed 100 amperes. Although the power associated with these pulses is very high, the duration of each pulse (often less than one microsecond) limits the energy of the induced arc to levels which are safe for the tube elements. Regardless of the type of pulses used for the spot-knocking, most users have found it prudent to avoid the application of negative pulses to the anode.
In recent years, improvements in the focusing of the electron spot on the screen have been achieved by the use of increasingly higher voltages on the focusing elements of both bipotential and tripotential types. Because of these higher operating potentials, it is often necessary to provide for spot-knocking, between the focus electrode G3 and the scree grid G2, for tripotential types, spot-knocking among the various focus grids G3, G4 and G5 is also believed to be desirable.
In another spot-knocking method, described in U.S. Pat. No. 4,052,776 to R. Maskell et al., very high amplitude RF bursts are added to the fluctuating DC pulses of relatively low amplitude which are used to spot-knock between G2 and G3. In this method, the fluctuating DC spot-knocking voltage pulses are introduced through the stem lead to the G3 and G5 of a tripotential gun, and the RF burst is introduced through the remainder of the stem leads which are electrically connected. Because the stem leads are close to one another, either the peak DC voltages must be maintained at relatively low values which are of limited effectiveness, or special precautions must be taken to prevent electrical breakdown among the external portions of the stem leads.
Yet another spot-knocking method is described in U.S Pat. No. 4,682,963 to Daldry et al. A two-step conditioning process is disclosed for a CRT having six grids. During normal operation, the G2 and G4 are interconnected to a relatively low voltage. The G3 and G5 focus electrodes are interconnected at a higher potential and the anode, G6, operates at the highest potential. A general conditioning includes applying high voltage DC to the anode and applying pulse voltages to the interconnected G2 and G4 electrodes, the heater, the cathode, and the G1 are interconnected and allowed to float. The G3 and G5 are interconnected to each other and also allowed to float. During the second step of the processing, the heater, the cathode and the G1 through G5 electrodes, inclusive, are connected to the pulse voltage with a high voltage DC applied to the anode.
While several of the above-described spot-knocking methods relates to six element electron guns (in addition to the heater and the cathode), none provides an adequate means for conditioning a double bipotential electron gun. A double bipotential gun structure typically has a heater, a cathode K, a control grid G1, a screen grid G2, a first focus electrode G3, a first anode G4, a second focus electrode G5 and a second anode G6. The first and second focus electrodes, G3 and G5, typically operate at about 7 kV and the first and second anodes, G4 and G6, typically operate at about 25 kV. In some six electrode electron gun individual output leads are provided on the mount stem for the G3 and G5 focus electrodes to permit spot-knocking between the anode electrodes and at least one of the focus electrodes with the other focus electrode electrically floating. Such a process is described in my copending patent application, serial number 214,554 filed Jun. 29, 1988 and entitled, "METHOD FOR SPOT-KNOCKING AN ELECTRON GUN MOUNT ASSEMBLY OF A CRT". However, not all double bipotential gun structures are provided with a mount stem having individual output leads for the G3 and G5 focus electrodes. Instead, the G3 and G5 focus electrodes are frequently internally interconnected and transverse arcing between the interconnecting lead, the other gun elements and the neck glass of the CRT envelope occurs during spot-knocking. As a result, fewer beneficial arcs are generated across the gaps between adjacent electrodes so that projections, burrs and/or particles which create field emission sites are not completely removed from the electrodes. The same incomplete spot-knocking can occur in any electron gun where there is a tendency for transverse arc to be initiated between the electron gun elements and the neck glass of the CRT.